Альтернативные биохимии: гипотезы о внеземной жизни

Possible alternative biochemicals used by life forms

Hypothetical types of biochemistry are forms of biochemistry agreed to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons it may be chemically similar, though it is also possible that there are organisms with quite different chemistries for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water. The possibility of life forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in synthetic biology and is also a common subject in science fiction. The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.

+ Overview of hypothetical types of biochemistry Type Basis Brief descriptionRemarks Alternative chirality biomolecules Alternative biochemistry Mirror image biochemistry Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth based life, amino acids are almost universally of the form and sugars are of the form. Molecules using amino acids or sugars are possible, though they would be incompatible with organisms using the opposing chirality molecules. Gram positive bacteria incorporate alanine into their Peptidoglycan layer, created through the actions of Racemases Ammonia biochemistry Non water solvents Ammonia based life Ammonia is relatively abundant in the universe and has chemical similarities to water. The possible role of liquid ammonia as an alternative solvent for life is an idea that goes back at least to 1954, when J. B. S. Haldane raised the topic at a symposium about life's origin. Arsenic biochemistry Alternative biochemistry Arsenic based life Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. Borane biochemistry (Organoboron chemistry) Alternative biochemistry Boranes based life Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing environment. Boron, however, is exceedingly rare in the universe in comparison to its neighbours carbon, nitrogen, and oxygen. On the other hand, structures containing alternating boron and nitrogen atoms share some properties with hydrocarbons. Cosmic necklace based biologyNonplanetary lifeNon chemical lifeIn 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky hypothesized that life composed of magnetic semipoles connected by cosmic strings could evolve inside stars. Extremophiles Alternative environment Life in variable environments It would be biochemically possible to sustain life in environments that are only periodically consistent with life as we know it. Heteropoly acid biochemistry Alternative biochemistry Heteropoly acid based life Various metals can form complex structures with oxygen, such as heteropoly acids. Hydrogen fluoride biochemistry Non water solvents Hydrogen fluoride based life Hydrogen fluoride has been considered as a possible solvent for life by scientists such as Peter Sneath. Hydrogen sulfide biochemistry Non water solvents Hydrogen sulfide based life Hydrogen sulfide is a chemical analog of water, but is less polar and a weaker inorganic solvent. Methane biochemistry (Azotosome) Non water solvents Methane based life Methane (CH4) is relatively abundant in the solar system and the universe, and is known to exist in liquid form on Titan, the largest moon of Saturn. Though highly unlikely, it is considered to be possible for Titan to harbor life. If so, it will most likely be methane based life. Non green photosynthesizers Other speculations Alternate plant life Physicists have noted that, although photosynthesis on Earth generally involves green plants, a variety of other colored plants could also support photosynthesis, essential for most life on Earth, and that other colors might be preferred in places that receive a different mix of stellar radiation than Earth. In particular, retinal is capable of, and has been observed to, perform photosynthesis. Bacteria capable of photosynthesis are known as microbial rhodopsins. A plant or creature that uses retinal photosynthesis is always purple. Shadow biosphere Alternative environment A hidden life biosphere on Earth A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. Silicon biochemistry (Organosilicon) Alternative biochemistry Silicon based life Like carbon, silicon can create molecules that are sufficiently large to carry biological information; however, the scope of possible silicon chemistry is far more limited than that of carbon. Silicon dioxide biochemistry Non water solvents Silicon dioxide based life Gerald Feinberg and Robert Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium. Sulfur biochemistry Alternative biochemistry Sulfur based life The biological use of sulfur as an alternative to carbon is purely hypothetical, especially because sulfur usually forms only linear chains rather than branched ones. Alternative nucleic acids Alternative biochemistry Different genetic storage Xeno nucleic acids (XNA) may possibly be used in place of RNA or DNA. XNA is the general term for a nucleic acid with an altered sugar backbone. Examples of XNA are TNA, which uses threose, HNA, which uses 1,5 anhydrohexitol, GNA, which uses glycol, CeNA, which uses cyclohexene, LNA, which utilizes a form of ribose that contains an extra linkage between its 4' carbon and 2' oxygen, FANA, which uses arabinose but with a single fluorine atom attached to its 2' carbon, and PNA, which uses, in place of sugar and phosphate, N (2 aminoethyl) glycine units connected by peptide bonds. In comparison, Hachimoji DNA changes the base pairs instead of the backbone. These new base pairs are P (2 Aminoimidazo[1,2a][1,3,5]triazin 4(1H) one), Z (6 Amino 5 nitropyridin 2 one), B (Isoguanine), and S (rS = Isocytosine for RNA, dS = 1 Methylcytosine for DNA).

A shadow biosphere is a hypothetical microbial biosphere of Earth that uses radically different biochemical and molecular processes than currently known life. Although life on Earth is relatively well studied, the shadow biosphere may still remain unnoticed because the exploration of the microbial world targets primarily the biochemistry of the macro organisms.

Perhaps the least unusual alternative biochemistry would be one with differing chirality of its biomolecules. In known Earth based life, amino acids are almost universally of the form and sugars are of the form. Molecules using amino acids or sugars may be possible; molecules of such a chirality, however, would be incompatible with organisms using the opposing chirality molecules. Amino acids whose chirality is opposite to the norm are found on Earth, and these substances are generally thought to result from decay of organisms of normal chirality. However, physicist Paul Davies speculates that some of them might be products of "anti chiral" life. It is questionable, however, whether such a biochemistry would be truly alien. Although it would certainly be an alternative stereochemistry, molecules that are overwhelmingly found in one enantiomer throughout the vast majority of organisms can nonetheless often be found in another enantiomer in different (often basal) organisms such as in comparisons between members of Archaea and other domains, making it an open topic whether an alternative stereochemistry is truly novel.

On Earth, all known living things have a carbon based structure and system. Scientists have speculated about the pros and cons of using atoms other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe. Sagan used the term "carbon chauvinism" for such an assumption. He regarded silicon and germanium as conceivable alternatives to carbon Norman Horowitz devised the experiments to determine whether life might exist on Mars that were carried out by the Viking Lander of 1976, the first U. S. mission to successfully land a probe on the surface of Mars. Horowitz argued that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. He considered that there was only a remote possibility that non carbon life forms could exist with genetic information systems capable of self replication and the ability to evolve and adapt.

The silicon atom has been much discussed as the basis for an alternative biochemical system, because silicon has many chemical similarities to carbon and is in the same group of the periodic table. Like carbon, silicon can create molecules that are sufficiently large to carry biological information. However, silicon has several drawbacks as a carbon alternative. Carbon is ten times more cosmically abundant than silicon, and its chemistry appears naturally more complex. Even though Earth and other terrestrial planets are exceptionally silicon rich and carbon poor (silicon is roughly 925 times more abundant in Earth's crust than carbon), terrestrial life bases itself on carbon. It may eschew silicon because silicon compounds are less varied, unstable in the presence of water, or block the flow of heat. Relative to carbon, silicon has a much larger atomic radius, and forms much weaker covalent bonds to atoms — except oxygen and fluorine, with which it forms very strong bonds. Alternatively, the weak bonds in silicon compounds may help maintain a rapid pace of life at cryogenic temperatures. Polysilanols, the silicon homologues to sugars, are among the few compounds soluble in liquid nitrogen. All known silicon macromolecules are artificial polymers, and so "monotonous compared with the combinatorial universe of organic macromolecules". Although not observed in nature, carbon–silicon bonds have been added to biochemistry under directed evolution (artificial selection): a cytochrome c protein from Rhodothermus marinus has been engineered to catalyze new carbon–silicon bonds between hydrosilanes and diazo compounds.

Boranes are dangerously explosive in Earth's atmosphere, but would be more stable in a reducing atmosphere. However, boron's low cosmic abundance makes it less likely as a base for life than carbon. Various metals, together with oxygen, can form very complex and thermally stable structures rivaling those of organic compounds; the heteropoly acids are one such family. Some metal oxides are also similar to carbon in their ability to form both nanotube structures and diamond like crystals (such as cubic zirconia). Titanium, aluminium, magnesium, and iron are all more abundant in the Earth's crust than carbon. Metal oxide based life could therefore be a possibility under certain conditions, including those (such as high temperatures) at which carbon based life would be unlikely. The Cronin group at Glasgow University reported self assembly of tungsten polyoxometalates into cell like spheres. By modifying their metal oxide content, the spheres can acquire holes that act as porous membrane, selectively allowing chemicals in and out of the sphere according to size. Sulfur reducing bacteria can utilize elemental sulfur instead of oxygen, reducing sulfur to hydrogen sulfide.)

Arsenic, which is chemically similar to phosphorus, while poisonous for most life forms on Earth, is incorporated into the biochemistry of some organisms. Some marine algae incorporate arsenic into complex organic molecules such as arsenosugars and arsenobetaines. Fungi and bacteria can produce volatile methylated arsenic compounds. Arsenate reduction and arsenite oxidation have been observed in microbes (Chrysiogenes arsenatis). Additionally, some prokaryotes can use arsenate as a terminal electron acceptor during anaerobic growth and some can utilize arsenite as an electron donor to generate energy. It has been speculated that the earliest life forms on Earth may have used arsenic biochemistry in place of phosphorus in the structure of their DNA. A common objection to this scenario is that arsenate esters are so much less stable to hydrolysis than corresponding phosphate esters that arsenic is poorly suited for this function. The authors of a 2010 geomicrobiology study, supported in part by NASA, have postulated that a bacterium, named GFAJ 1, collected in the sediments of Mono Lake in eastern California, can employ such 'arsenic DNA' when cultured without phosphorus. They proposed that the bacterium may employ high levels of poly β hydroxybutyrate or other means to reduce the effective concentration of water and stabilize its arsenate esters. Science writer Carl Zimmer contacted several scientists for an assessment: "I reached out to a dozen experts Almost unanimously, they think the NASA scientists have failed to make their case". Other authors were unable to reproduce their results and showed that the study had issues with phosphate contamination, suggesting that the low amounts present could sustain extremophile lifeforms. Alternatively, it was suggested that GFAJ 1 cells grow by recycling phosphate from degraded ribosomes, rather than by replacing it with arsenate.

Methane (CH4) is a simple hydrocarbon: that is, a compound of two of the most common elements in the cosmos: hydrogen and carbon. It has a cosmic abundance comparable with ammonia. Water is a stronger solvent than the hydrocarbons, enabling easier transport of substances in a cell. However, water is also more chemically reactive and can break down large organic molecules through hydrolysis. Possible evidence for this form of life on Titan was identified in 2010 by Darrell Strobel of Johns Hopkins University; a greater abundance of molecular hydrogen in the upper atmospheric layers of Titan compared to the lower layers, arguing for a downward diffusion at a rate of roughly 1025 molecules per second and disappearance of hydrogen near Titan's surface. As Strobel noted, his findings were in line with the effects Chris McKay had predicted if methanogenic life forms were present. The same year, another study showed low levels of acetylene on Titan's surface, which were interpreted by Chris McKay as consistent with the hypothesis of organisms reducing acetylene to methane. He noted that even a non biological catalyst effective at 95 K would in itself be a startling discovery. An analysis of data obtained using the Atacama Large Millimeter / submillimeter Array (ALMA), completed in 2017, confirmed substantial amounts of acrylonitrile in Titan's atmosphere. Later studies questioned whether acrylonitrile would be able to self assemble into azotozomes.

Hydrogen fluoride (HF), like water, is a polar molecule, and due to its polarity it can dissolve many ionic compounds. At atmospheric pressure, its melting point is , and its boiling point is ; the difference between the two is a little more than 100 K. HF also makes hydrogen bonds with its neighbor molecules, as do water and ammonia. It has been considered as a possible solvent for life by scientists such as Peter Sneath and Carl Sagan. However, hydrogen fluoride is cosmically rare, unlike water, ammonia, and methane.

Hydrogen sulfide is the closest chemical analog to water, but is less polar and is a weaker inorganic solvent. Hydrogen sulfide is quite plentiful on Jupiter's moon Io and may be in liquid form a short distance below the surface; astrobiologist Dirk Schulze Makuch has suggested it as a possible solvent for life there. On a planet with hydrogen sulfide oceans, the source of the hydrogen sulfide could come from volcanoes, in which case it could be mixed in with a bit of hydrogen fluoride, which could help dissolve minerals. Hydrogen sulfide life might use a mixture of carbon monoxide and carbon dioxide as their carbon source. They might produce and live on sulfur monoxide, which is analogous to oxygen (O2). Hydrogen sulfide, like hydrogen cyanide and ammonia, suffers from the small temperature range where it is liquid, though that, like that of hydrogen cyanide and ammonia, increases with increasing pressure.

Silicon dioxide, also known as silica and quartz, is very abundant in the universe and has a large temperature range where it is liquid. However, its melting point is , so it would be impossible to make organic compounds in that temperature, because all of them would decompose. Silicates are similar to silicon dioxide and some have lower melting points than silica. Feinberg and Shapiro have suggested that molten silicate rock could serve as a liquid medium for organisms with a chemistry based on silicon, oxygen, and other elements such as aluminium.

The genetic code may have evolved during the transition from the RNA world to a protein world. The Alanine World Hypothesis postulates that the evolution of the genetic code (the so called GC phase) started with only four basic amino acids: alanine, glycine, proline and ornithine (now arginine). The evolution of the genetic code ended with 20 proteinogenic amino acids. From a chemical point of view, most of them are Alanine derivatives particularly suitable for the construction of α helices and β sheets basic secondary structural elements of modern proteins. Direct evidence of this is an experimental procedure in molecular biology known as alanine scanning. A hypothetical "Proline World" would create a possible alternative life with the genetic code based on the proline chemical scaffold as the protein backbone. Similarly, a "Glycine World" and "Ornithine World" are also conceivable, but nature has chosen none of them. Evolution of life with Proline, Glycine, or Ornithine as the basic structure for protein like polymers (foldamers) would lead to parallel biological worlds. They would have morphologically radically different body plans and genetics from the living organisms of the known biosphere.

In 2007, Vadim N. Tsytovich and colleagues proposed that lifelike behaviors could be exhibited by dust particles suspended in a plasma, under conditions that might exist in space. Computer models showed that, when the dust became charged, the particles could self organize into microscopic helical structures, and the authors offer "a rough sketch of a possible model of helical grain structure reproduction".

In 2020, Luis A. Anchordoqu and Eugene M. Chudnovsky of the City University of New York hypothesized that cosmic necklace based life composed of magnetic monopoles connected by cosmic strings could evolve inside stars. This would be achieved by a stretching of cosmic strings due to the star's intense gravity, thus allowing it to take on more complex forms and potentially form structures similar to the RNA and DNA structures found within carbon based life. As such, it is theoretically possible that such beings could eventually become intelligent and construct a civilization using the power generated by the star's nuclear fusion. Because such use would use up part of the star's energy output, the luminosity would also fall. For this reason, it is thought that such life might exist inside stars observed to be cooling faster or dimmer than current cosmological models predict.

Frank Drake suggested in 1973 that intelligent life could inhabit neutron stars. Physical models in 1973 implied that Drake's creatures would be microscopic.